diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index e2ae07444..d8f73ae96 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -1396,7 +1396,7 @@ int MBEDTLS_DEPRECATED mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,
  *                  set in \p key.
  * \return          Another negative error code on different kinds of failure.
  */
-int mbedtls_ecp_write_key_ext(mbedtls_ecp_keypair *key,
+int mbedtls_ecp_write_key_ext(const mbedtls_ecp_keypair *key,
                               size_t *olen, unsigned char *buf, size_t buflen);
 
 /**
diff --git a/library/ecp.c b/library/ecp.c
index 0dadaeaac..427059bb5 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -3335,7 +3335,7 @@ cleanup:
 }
 #endif /* MBEDTLS_DEPRECATED_REMOVED */
 
-int mbedtls_ecp_write_key_ext(mbedtls_ecp_keypair *key,
+int mbedtls_ecp_write_key_ext(const mbedtls_ecp_keypair *key,
                               size_t *olen, unsigned char *buf, size_t buflen)
 {
     size_t len = (key->grp.nbits + 7) / 8;